It is quite common on construction sites to use devices for detecting underground structures before or while digging. Such structures are often occurring in form of services for electricity, gas, fuel, water, or communication data among other conductive underground structures. Although the location of most of these services is already known from a surveyor's plan of the site, their locations can have uncertainties, or there could be additional services that are not mentioned therein. Often, they are also simply overlooked by the operator of an earth moving machine during work.
The prevention of damage to underground structures while digging in a trench or in areas being excavated is an important task. As damage to a service can cause serious impact and costs, these additional measurements are taken in order to be able to detect the proximity of such services on the site before or while excavating. Devices for this purpose are known as Cable Avoidance Tools also-called CAT. An embodiment of such a device is for example described in EP 2 362 241.
One way to locate underground services is to detect electromagnetic fields sent out by the service itself. In order to do this the services must have a naturally occurring electrical signal witch emits a field that is detectable above the ground. This works well for active power supplies, but for example a wiring system of switched off street lights, unused or low-voltage communication cables, gas- or water-pipes can hardly be detected in this way.
In order to avoid this limitation, several developments were made to improve the detection and to be able to detect different types of services as well. U.S. Pat. No. 5,194,812, U.S. Pat. No. 4,600,356, U.S. Pat. No. 5,592,092 and U.S. Pat. No. 6,437,726 are referring to such approaches. The underground structures need to emit electromagnetic fields within a usable frequency range and which are strong enough to be detectable above the surface. As the fields have to be caused by naturally occurring electrical signals, the usability of this method is limited to live power lines and some communication cables.
U.S. Pat. No. 4,438,401, JP 2003 227878 and DE 27 35 344 are disclosing systems wherein metallic services with no naturally occurring signals are directly connected to a signal-generator. In this way an electrical signal can be conducted to the service, and therefore it is possible to detect it by its electromagnetic field. For doing so the service has to be located and connected somewhere near the construction site. This can be quite a difficult task, since some services are hidden, not accessible or even live wires and therefore cannot be connected to. These systems work well for lots of metallic services, but the detection is still limited to services containing electrically conducting materials. For example, water-conduits made out of plastic are hardly detectable in this way.
In EP 2 278 358 the signal to be detected is an artificially generated signal from a signal source, which is introduced into soil by at least two electrodes (e.g. earth spikes, tent-pegs, etc.) which are connected to the signal source. For doing so, there is at least one long cable required, as good detection results require a placement of the electrodes where they are spaced apart at least five meters, preferably some tens or even hundreds of meters. The advantage of this method is that almost any service which provides a better electrical conductivity than soil can be detected by such a system.
A disadvantage is the long cable which is required to supply the signals to the soil. Such a long cable is quite difficult to handle and requires a big bulky cable reel. Especially at worksites,—having lots of machinery, vehicles, workers and other traffic—a laying of a long cable above ground can be difficult and problematic, as it will probably impede the workflow. The cable might also easily get damaged by some machinery or worker or can cause stumbling. A proper laying out of such a cable for connecting an earth spike for signal injection is a time consuming preparation job for the actual detection task. Also the removing of the long cable and a proper storage on the cable reel, often combined with a cleaning of the cable from worksite dirt, can take a long time. The establishing and removing of the earth current setup can often take even longer than the actual detection.